1. Field of the Invention
The present invention relates to a scanning system, and more particularly, to an near electric-field probe capable of measuring vectorial electric field by using short optical pulse and an ultrafast photoconductive switch.
2. Description of the Prior Art
Continuing advances in the development of novel materials and modern semiconductor devices/circuits have pushed the operating frequency of quantum-electronic and opto-electronic devices/circuits over multi-hundred gigahertz frequency region and have brought the millimeter-wave circuits into real world. The integrity of the devices/circuits in turn is becoming more and more increasing. On the other hand, high-speed devices/circuits have been facing unexpected phenomena and problems related to electro-magnetic field therein. Accordingly, needs for a novel diagnostic system to cope with those kind of phenomena and problems are also increasing. Moreover, there have been needs not only for diagnosis for electrical characteristics of the devices/circuits but for spectroscopic technologies reaching even far infra-red region in order to give a diagnosis of various electric and optical characteristics of materials.
One of the most popular methods for the diagnosis is to measure near-field with high spatial resolution. A probe or detector for use in measuring the near electric-field is required to have wide range of bandwidth and high temporal resolution for measuring various frequency components as well as to be small enough to detect field at any measured point. Also required are a flexible polarization and a high sensitivity(signal to noise ratio) to achieve precise measurement. In addition, a loading effect and an invasiveness should be low enough to minimize the distortion due to the detector or the peripheral devices with respect to a device under test(DUT).
Since 1980s, a variety of scanning probe microscopes such as a scanning tunnel microscope(STM), a scanning force microscope(SFM) and a scanning optical mocroscope(SOM) have been developed. Such scanning probe microscopes have a spatial resolution up to a degree of elements, and therefore, are feasible for measurement of ultrafast high-integrated devices/circuits. The electrical measurement frequencies of such microscopes are limited depending on the electronic parts employed therein. For instance, they exhibit rather low measurement sensitivity due to a tunneling current introduced thereto as for the STM, due to deviation of a cantilever as for the SFM, and due to exploiting an evanescent-wave coupling as for the SOM.
In an effort to overcome the reduction in the measurement bandwidth and low measurement sensitivity resulting from employment of these peripheral devices, a photoconductive sampling technology has been developed, in which picosecond electrical signals are measured by introducing short optical pulses into an ultrafast photoconductive switch. The photoconductive sampling technology has advantages that it can measure sub-picosecond signals with high sensitivity by exploiting low frequency devices. The photoconductive sampling technology involves a photoconductive switch which is a kind of a metal-semiconductor-metal(MSM) photodetector. If a laser beam with short pulse duration(e.g., a laser beam having less than 100 fs can be generated using the mode-locked Ti:Sapphire laser) is introduced to the photoconductive switch, carriers are generated in the photoconductive region of the photoconductive switch. Measured signals can be detected from the behavior of the carriers which display the behavior in conformity to the electrical signal under measurement. An electrical pulse with very narrow duration may be generated when the lifetime of the carriers is very short. Herein, a photoconductor having carriers exhibiting very short lifetime is called an ultrafast photoconductive body. LT-GaAs(Low-temperature-grown GaAs) and SOS(Silicon-on-Sapphire) are examples of the ultrafast photoconductive body, especially with carriers having lifetime of sub-picoseconds. There may be two methods for measuring electrical signals with resolution of sub-picoseconds on the basis of a photoconductive sampling principle. One is a pump-probe measurement(PPM) and the other is a time-equivalent sampling(TES).
According to the PPM, a short pulse laser beam generated from a laser is splitted into two beams: a pump laser pulse beam and a probe laser pulse beam. The pump laser pulse beam is introduced onto a first ultrafast photoconductive switch(or photodetector) to generate an electrical pulse signal, while the probe laser pulse beam is introduced to a second ultrafast photoconductive switch via a fine translation stage or an optical delay-line, to detect the electrical pulse signal generated at the first photoconductive switch. Herein, where a device under test(DUT; e.g., electrical device or circuit) is placed in-between the first photoconductive switch and the second photoconductive switch, the second photoconductive switch may present an impulse response of the DUT in response to input of the electrical pulse signal from the first photoconductive switch, wherein the impulse response is regarded as a measured signal. The measured signal includes a signal presenting cross-correlation between the electrical pulse from the second photoconductive switch and the to-be-measured signal with respect to a time delay between the pump laser pulse beam and the probe laser pulse beam. That is, the second ultrafast photoconductive switch measures electrical characteristics of the DUT by sampling electrical signal proportional to the to-be-measured signal from a time delay of the first ultrafast photoconductive switch.
As described above, the PPM is basically a method for measuring pulse response. The TES, on the other hand, is a method for measuring ultra high frequency signal operating with respect to a single frequency in the steady state. The TES requires synchronization between repetition rate of laser pulse beams(flaser) and the operating frequency(fm) of the DUT. In other words, signal fIF=ABS(fm−n·flaser) obtained by mixing the electrical signal flaser obtained by introducing laser pulses to the first photoconductive switch(or photodetector) and to-be-measured signal with frequency of fm, is used for triggering a lock-in amplifier(or an oscilloscope or a spectrum analyzer). Then, sampling the electrical signal by activating the second ultrafast photoconductive switch with another probe laser pulse beam generated at the laser, results in signals equivalent to the to-be-measured signal at the frequency of fIF.
Measurement of electrical signal by the photoconductive sampling method was first proposed by D. H. Auston, and this method is also called an “On-wafer Measurement Method.” FIG. 1 is a schematic diagram of a set-up for measuring electrical signal propagating on a conducting line by using the photoconductive sampling method. The on-wafer measurement method employs a photoconductive switch to be used as an electrical pulse signal generator 110 and an electrical signal detector 120. More specifically, the conducting line and a first photoconductive switch 110 connected thereto are placed on the ultrafast photoconductive body and dc voltage is applied to the first photoconductive, switch 110, a pump laser pulse beam generated from a short pulse laser system 140 is applied thereto in order to generate the electrical pulse signal propagated through the conducting line, consecutively a probe laser pulse beam is guided to pass through an optical delay-line before being applied to a second photoconductive switch 120 and a third photoconductive switch (not shown, but positioned at the side of the electrical signal generator 110), and finally, the electrical signal passing through or being reflected from the DUT may be detected. Therefore, according to the on-wafer measurement method, electrical characteristics of the DUT which are positioned between photoconductive switches may be detected. In order to overcome disadvantages of the on-wafer measurement method that the photoconductive switches must be associated with the DUT on the ultrafast photoconductive body, the photoconductive switches are associated with electrical contact probe to measure electrical characteristics of an DUT. This method, however, still bears limitations that it can measure only a resultant scalar voltage/current not a vectorial component which collectively constitutes electromagnetic wave.
Another exemplary method on the basis of the photoconductive sampling principle is a terahertz system. A so-called terahertz radiation refers to a phenomenon that when short laser pulse is introduced onto a photoconductive switch biased with dc voltage, not only electrical pulse signal is generated and propagated through an electrical line connected to the photoconductive switch in the wake of generated carriers and but also electromagnetic wave which is the derivative of the electrical pulse signal is radiated. A system exploits the terahertz radiation is called terahertz system. A terahertz signal radiated from a terahertz transmitter is measured by the photoconductive sampling principle in order to identify characteristics of various materials and to perform imaging(also called a terahertz imaging), wherein the terahertz transmitter introduces laser pulse beam onto a photoconductive switch biased with dc voltage to radiate terahertz wave and another photoconductive switch positioned on the ultrafast photoconductive body through an optical delay-line detects the responsive terahertz wave.
FIG. 2 is a schematic diagram of the terahertz system. As shown in FIG. 2, a pump laser pulse beam generated from the short pulse laser system 240 is introduced to a transmitter including a first photoconductive switch 210 which is biased with dc voltage in order to generate terahertz electromagnetic wave, and on the other hand, a probe laser pulse beam is introduced to a receiver including a second photoconductive switch 220 through the optical delay-line 250 to measure the terahertz electromagnetic wave. A terahertz electromagnetic wave that has been radiated from the first photoconductive switch 210 of the transmitter and then transmitted through or reflected on the DUT 230 is measured by the second photoconductive switch 220 of the receiver. Accordingly, the electrical characteristics of the DUT 230 positioned between the terahertz transmitter 210 and the terahertz receiver 220 can be measured. The terahertz system, however, also bears limitations that it exhibits rather low spatial resolution and is inappropriate for use in near-field measurement due to relatively large sizes of the transmitter/receiver and due to various auxiliary devices for collecting the radiated terahertz electromagnetic wave.
Meanwhile, when carriers are generated by introducing short laser pulse to unbiased ultrafast photoconductive switch placed in the free space, the carriers displays movements affected by ambient electromagnetic field. The movement of the carriers solely depends on the component of the electromagnetic field that is parallel to the ultrafast photoconductive switch. Accordingly, components of electromagnetic field can be distinctively observed under the photoconductive sampling principle employing the ultrafast photoconductive switch.